The elevated temperature strength of precipitation hardened nickel-base superalloys has encouraged their widespread use in aircraft gas turbine engine components. For components such as turbine disks and seals, the development of improved nickel-base superalloy materials has historically been aimed at achieving higher tensile, creep, and stress-rupture strengths. Improvements in these properties can permit increased engine performance and efficiency. Within recent years, there has also been a growing demand for greater fatigue resistance. This need has been driven by the strong desire to precisely define component life and establish engine durability. Improvements in fatigue resistance reduce life cycle costs and the frequency of in-service inspections. It has also been observed that the sensitivity to fatigue failure usually increases as material strength levels and component operation temperatures are raised. In some cases, critical disk components have even become fatigue limited, prompting much attention to fatigue property characterization and improvement during alloy and process development programs.
Current design methodologies for turbine disks typically use fatigue properties, as well as conventional tensile, creep, and stress-rupture properties for sizing and life analyses. In many instances, the most suitable means of quantifying fatigue behavior for these analyses is through the determination of crack growth rates as described by linear elastic fracture mechanics (LEFM). Under LEFM, the rate of fatigue crack propagation per cycle (da/dN) is a single-valued function which can be described by the stress intensity range, delta K, defined as Kmax-Kmin. Delta K serves as a scale factor to define the magnitude of the stress field at a crack tip and is given in general form as delta K=f(stress, crack length, and geometry).
For improved disks, it has become desirable to develop and use materials which exhibit slow, stable crack growth rates, along with high tensile, creep, and stress-rupture strengths. The development of new nickel-base superalloy materials which offer simultaneously the improvements in, and/or an appropriate balance of tensile, creep, stress-rupture, and fatigue crack growth resistance, essential for advancement in the aircraft gas turbine art, presents a sizeable challenge. The challenge results from the competition between desirable microstructures, strengthening mechanisms, and composition features. The following are typical examples of competition: (1) A fine grain size, for example a grain size smaller than about ASTM 10, is typically desirable for improving tensile strength while a coarse grain size, for example a grain size greater than about ASTM 10, is desirable for improving creep, stress-rupture, and crack growth resistance; (2) Small shearable precipitates are desirable for improving fatigue crack growth resistance under certain conditions, while shear resistant precipitates are desirable for high tensile strength; (3) High precipitate-matrix coherency strain is desirable for high tensile strength while low coherency strain is typically desirable for good stability, creep-rupture resistance, and probably good fatigue crack growth resistance; (4) Generous amounts of refractory elements such as W, Ta, or Nb, can significantly improve strength but must be used in moderate amounts to avoid unattractive increases in alloy density.
Once compositions exhibiting attractive mechanical properties have been identified in laboratory scale investigations, there is also a considerable challenge in successfully transferring this technology to large full-scale production hardware, for example turbine disks of diameters up to, but not limited to, 25 inches. These problems are well known in the metallurgical arts. There has not been reported a nickel-base superalloy material which provides an enhanced, beneficial balance of strength, creep, stress-rupture, and fatigue crack growth resistance, or an alloy-process combination, that would allow full scale production implementation and practical engineering use of such material.